I've tried at least 80 Mudshells and never seen a Rare. Finally gave up and stoned one. And even Wowhead doesn't know yet what breeds there are but almost all my battles have been against B/B and just the occasional H/B.

No idea if it's just that the RNG gods hate me or that Blizzard having fun with pet tuning, but from experience does seem like the rarity tuned differently for different pets.